1. Technical Field
The present invention relates to a method for forming an identification mark on a refractory (high melting point) material single crystal substrate and particularly to a method for forming an identification mark on a refractory material single crystal substrate using a laser beam.
2. Description of the Related Art
In recent years, refractory materials, such as silicon carbide (SiC), gallium nitride (GaN), aluminum nitride (AlN), zinc oxide (ZnO), have been receiving attention as new materials for semiconductor devices or as new growth substrates for semiconductors. For example, the silicon carbide semiconductor has a larger dielectric breakdown electric field, a faster saturated drift velocity of electrons, and a greater thermal conductivity than those of the silicon semiconductor. Thus, research and development have been intensively carried out for realizing a power device which is capable of a large current operation at a high temperature and at a high speed with the use of a silicon carbide semiconductor as compared with conventional silicon devices. Among others, motors for use in electric motorcycles, electric vehicles, and hybrid vehicles are AC-driven or inverter-controlled, and therefore, development of efficient switching devices for such uses has been receiving attention. To realize such power devices, a silicon carbide single crystal substrate for epitaxial growth of a high-quality silicon carbide semiconductor layer is necessary.
Demands for blue laser diodes which are used as a light source for recording data at a high density and white diodes which are used as a light source in place of a fluorescent lamp or an incandescent bulb have been growing. Such light-emitting devices are manufactured using a gallium nitride semiconductor, and in some cases, a wide-bandgap compound semiconductor substrate such as a silicon carbide single crystal substrate is used as the substrate for formation of a high-quality gallium nitride semiconductor layer. Therefore, there is demand for a wide-bandgap compound semiconductor substrate which is used as a substrate for manufacture of a semiconductor device for which demand is expected to undergo a large growth in the future, such as a silicon carbide semiconductor device, a gallium nitride semiconductor device, etc.
To a semiconductor substrate which is used for manufacture of a semiconductor device, information for identification is provided as an identification mark for identifying semiconductor substrates and managing the process conditions of the manufacture process through which they have undergone for each of the semiconductor substrates. Usually, the identification mark has a size which is perceivable by a human eye. However, in other cases, the identification mark is imaged by a camera or the like and image-processed so as to be detected by a semiconductor manufacturing apparatus or the like.
In forming an identification mark on a semiconductor substrate, a laser beam is usually used. The semiconductor in a region irradiated with a laser beam is melted and evaporated, whereby a recessed portion is formed in the surface of the semiconductor substrate. The recessed portion constitutes an identification mark. According to the depth of this recessed portion, the method for forming an identification mark is generally divided into two types. Specifically, formation of an identification mark with a recessed portion depth of about 0.1 μm to 5 μm is referred to as “soft marking”, and formation of an identification mark with a recessed portion depth of about 5 μm to 100 μm is referred to as “hard marking”. Also, in some cases, the identification mark is constituted of a recessed portion which is in the form of an independent dot, and in other cases, the identification mark is constituted of one or more linear grooves.
The above-described refractory semiconductor is a new semiconductor material and has a higher melting point than other semiconductor materials which are widely employed, such as silicon, gallium arsenide, etc. Therefore, it is generally difficult to form a desirable identification mark on a refractory single crystal substrate under the conditions that are suitable for formation of an identification mark on a silicon substrate. Japanese Laid-Open Patent Publication No. 2006-43717 (hereinafter, referred to as Patent Document 1) discloses the technique of forming an identification mark which has an excellent visibility, which is realized by irradiating a silicon carbide single crystal substrate with pulsed laser light which has a predetermined pulse shape such that the silicon carbide is melted, whereby a slightly-recessed region is formed which contains a greater amount of carbon or silicon.